Presently, parts made from commercial, fiber-reinforced composites or laminates use vacuum-bag or press molding of plies of prepregs containing fibers and polymeric resins. Historically these composites were composed of thermosetting resins. The individually impregnated plies or layers of prepregs were consolidated by the application of heat and pressure by vacuum-bag molding and compaction in an autoclave. In recent years, thermoplastic polymers have been developed which have some benefits over thermoset polymers. The main differences between the two systems are that thermosets require curing, while thermoplastics only require melting, thus saving time, and thermoplastics offer improved damage tolerance.
The aircraft industry, for example, forms fiber-reinforced, thermoplastic resin matrixes into composite parts or structures for the construction repair of aircraft sections such as wing parts. This involves processing the composites at elevated temperatures and pressures for a set length of time. The length of time required to properly consolidate a part determines the production rate for that part and it is most desireable to achieve the best consolidation in the shortest amount of time. High temperatures are required to reach the melting point of the resin and transform the individual plies of prepreg into a fully consolidated solid laminate. High consolidation pressures, from about 100 to 300 psi, through the use of an autoclave, are needed to provide the force required to consolidate the plies and remove any defects, such as voids found in a laminate and driving portions of the volatiles into solution. Voids are usually caused by the inability of the resin to displace air from the fiber surface during the time the fibers are coated with the liquid resin. Voids may also be caused by air bubbles and volatiles entrapped within and between the plies before consolidation. Volatiles may also appear because of moisture in the prepreg or volatile material in the resin, etc.
In processing thermoplastic resin composites, comprising a poly-ether-ether-ketone or polyether sulfone etc., the industry has always relied on an autoclave or a press, to provide the required pressures. Autoclaves and presses are large, expensive pieces of equipment and cannot always provide void-free laminates or composites. The size, weight and cost of an autoclave prevents it from being used in the field at maintenance locations or smaller manufacturing sites. The lack of this important equipment limits these sites in being able to fabricate aircraft sections and/or repair and maintain aircraft in the field where there is a need to custom make void-free and volatile-free laminates.
For instance, one sealing system used for the manufacture of composite or laminated structures in a mold is disclosed in U.S. Pat. No. 4,681,651 to Brozovic et al. entitled "Vacuum Bag Sealing System". This patent discloses a sealing system for composite structures that must then be placed in an autoclave for final curing. Here, the system uses a single vacuum bag and comprises a base plate for mounting the mold and sheets of composite material, a first TEFLON, or other non-adhering substance, coated sealing surface bonded to said plate, a vacuum bag having another sealing surface and adhesive means for detachably sealing said first and said second surfaces to form an airtight seal. The single bag system is intended for use in an oven or autoclave cure.
Another patent, U.S. Pat. No. 4,504,341, entitled "Fabricating Shaped Laminated Transparencies" by John Radzwill et al. describes a method of simultaneously shaping and laminating rigid plies of acrylic or polycarbonate polymers with an interlayer of either polyurethane or polyvinyl butyral. The patent teaches that layers to be laminated are assembled into a stacked array and placed in a first flexible cell. Once the cell is completely formed it is evacuated by connection to a first vacuum source and it is then laid over the upper facing surface of a vacuum mold. A second, flexible bag is placed over the cell, the cell heated to a temperature of 210.degree. F., and a second vacuum drawn. Once the cell has conformed to the shape of the vacuum mold, the temperature is reduced until the shape of the assembly is set and then the various vacuums discharged simultaneously. This method, however, does not allow for the use of thermoplastic prepregs. Moreover, this method utilizes a large vacuum differential and therefore allows edges of the individual plies to be pinched off, thereby preventing complete removal of volatiles as soon as the second vacuum is drawn. In addition, this system will only work with polymeric prepregs at temperatures of approximately 210.degree. F.